One useful aspect of semiconductor-on-insulator (SOI) structures is that they permit the use of high-voltage SOI devices, such as diodes, field effect transistors (FETs), thyristors, and bipolar transistors. Still higher voltages may be achieved by connecting a plurality of such devices in series. However, doing so increases the difference in voltage potential between the device and an underlying substrate. This difference increases in each downstream device in the series. As such, the type and number of high-voltage SOI devices that may be connected in series is ultimately limited by the difference in voltage potential between the terminal device and its underlying substrate. Too great a difference in voltage potential will result in degradation of the breakdown voltage (Vbr) of the series device, making the device “leaky.” This can adversely impact the efficiency of the series device, sometimes to a degree that the series device fails. For example, in the case of five high-voltage (i.e., 30 V) diodes connected in series, the voltage at the terminal diode would theoretically be 150 V. However, at or near the terminal diode, this may result in too great a difference in voltage potential with the substrate, resulting in the voltage at the terminal diode being less than 150 V.
FIG. 1 shows an integrated circuit 100 including a substrate 10, a buried oxide (BOX) layer 20, and a semiconductor layer 30. Within semiconductor layer 30 are a plurality of HV SOI devices, here shown as diodes 40A-40D, connected in series. Diode 40A comprises a p-doped portion 42A and n-doped portion 44A. For the sake of clarity, the p-doped portions and n-doped portions of diodes 40B-D are not labeled, but are similar to p-doped portion 42A and n-doped portion 44A of diode 40A.
As can be seen in FIG. 1, a difference in voltage potential 41A between diode 40A and substrate 10 is less than a difference in voltage potential 41B between diode 40B and substrate 10. A difference in voltage potential 41C between diode 40C and substrate 10 is greater than difference in voltage potential 41B, and a difference in voltage potential 41D between diode 40D (the terminal diode) and substrate 10 is greater still. As noted above, difference in voltage potential 41D may be so great that the breakdown voltage degrades, resulting in voltage leakage.